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Parabolized stability analysis of jets from serrated nozzles

Published online by Cambridge University Press:  15 January 2016

Aniruddha Sinha*
Affiliation:
Mechanical Engineering, California Institute of Technology, Pasadena, CA 91125, USA Aerospace Engineering, Indian Institute of Technology Bombay, Powai 400 076, India
Kristján Gudmundsson
Affiliation:
Mechanical Engineering, California Institute of Technology, Pasadena, CA 91125, USA Quintiq, Utopialaan 25, 5232 CD, ’s-Hertogenbosch, The Netherlands
Hao Xia
Affiliation:
Aeronautical Engineering, Loughborough University, Leicestershire LE11 3TU, UK
Tim Colonius
Affiliation:
Mechanical Engineering, California Institute of Technology, Pasadena, CA 91125, USA
*
Email address for correspondence: as@aero.iitb.ac.in

Abstract

We study the viscous spatial linear stability characteristics of the time-averaged flow in turbulent subsonic jets issuing from serrated (chevroned) nozzles, and compare them to analogous round jet results. Linear parabolized stability equations (PSE) are used in the calculations to account for the non-parallel base flow. By exploiting the symmetries of the mean flow due to the regular arrangement of serrations, we obtain a series of coupled two-dimensional PSE problems from the original three-dimensional problem. This reduces the solution cost and manifests the symmetries of the stability modes. In the parallel-flow linear stability theory (LST) calculations that are performed near the nozzle to initiate the PSE, we find that the serrated nozzle reduces the growth rates of the most unstable eigenmodes of the jet, but their phase speeds are approximately similar. We obtain encouraging validation of our linear PSE instability wave results vis-à-vis near-field hydrodynamic pressure data acquired on a phased microphone array in experiments, after filtering the latter with proper orthogonal decomposition (POD) to extract the energetically dominant coherent part. Additionally, a large-eddy simulation database of the same serrated jet is investigated, and its POD-filtered pressure field is found to compare favourably with the corresponding PSE solution within the jet plume. We conclude that the coherent hydrodynamic pressure fluctuations of jets from both round and serrated nozzles are reasonably consistent with the linear instability modes of the turbulent mean flow.

Type
Papers
Copyright
© 2016 Cambridge University Press 

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